To install you need OpenIV, then just open vehicles.img and make sure you are on edit mode and then drag and drop the files in there.
Then open handling.dat and find Willard and change the line to:

WILLARD 1630.0 0.00 85 0.0 0.04 -0.5 0.0 4 0.137 1.0 155.0 0.17 0.650 0.30 35.0 1.10 0.95 16.8 0.15 0.4830 2.0 0.8 0.8 0.14 -0.16 0.0 0.44 1.0 0.6 1.2 1.5 0.0 18000 440000 0 0

Optionally you can use the old skin with red flames instead of the black skin with orange lines.

Enjoy.

Credits

Amistech Games- for making the car
GTA Mods Del Norte- for porting the car over to SA
CarCrasher730- for porting the car from SA to IV

Changelog:

0.1 
Fixed door collisions